As semiconductor devices become highly integrated, a MOS transistor begins to have a source/drain region of shallow junction. In order to increase reliability of the MOS transistor, a technique of forming LDD-type source/drain regions has been widely used. A gate spacer is formed at the sidewall of a gate electrode to form a LDD-type source/drain region. A silicon nitride layer having etch selectivity with a silicon oxide layer composing an interlayer insulation layer may be used to form a self-aligned contact suitable for a highly integrated semiconductor device as a conventional material layer to form a gate spacer.
To organize a circuit of a SRAM or non-memory semiconductor device, a gate electrode of one transistor may be directly connected with a source/drain region of another transistor. If the gate electrode and the source/drain region are closely arranged, a shared contact is formed for electrical connection instead of separate contacts.
FIG. 1 is an example of a layout of the SRAM cell using shared contacts.
FIGS. 2 through 4 illustrate cross-sectional views displaying a method of forming an LDD-type semiconductor device using a conventional shared contact.
Referring to FIG. 2, an isolation layer 2 is formed in a desired region of a semiconductor substrate 1 to isolate an active region. A gate insulation layer 3 is formed on the active region, and then a polysilicon layer 5 and a metal silicide layer 7 are sequentially stacked over the entire surface of the semiconductor substrate, including the gate insulation layer 3, thereby forming a gate electrode layer. The gate electrode layer is then patterned to form a couple of gate patterns 8 crossing a top part of a desired region of the gate insulation layer 3. Because the gate insulation layer 3 may be over-etched, the active region of both sides of the gate pattern 8 may be exposed.
Impurity ions are implanted in the active region in a dose amount of 1×1012 to 1×1013 atoms/cm2 by using the gate pattern 8 as an ion-implantation mask. A low-concentration source/drain region 9 is formed in the active region located on both sides of each gate pattern 8. Next, a gate spacer 11 of silicon nitride is formed on the sidewall of the gate pattern 8. Impurity ions are implanted in the low-concentration source/drain region 9 in a dose amount of 1×1015 to 5×1015 atoms/cm2 using the gate spacer 11 and the gate patterns 8 as ion-implantation masks. Thus, a high-concentration source/drain region 13 is formed, which has a higher impurity concentration than the low-concentration source/drain region 9. The low-concentration source/drain region 9 and the high-concentration source/drain region 13 compose LDD-type source/drain regions 15.
Referring to FIG. 3, oxide layer etching is performed with respect to the resultant structure, thereby exposing a silicon surface where the LDD-type source/drain regions 15 are formed. An etch-stopping layer 17 is formed over the entire surface. The etch-stopping layer 17 is made of a material layer having an etch selectivity with respect to silicon oxide. An interlayer insulation layer 19 is formed over the entire surface of the semiconductor substrate, including the etch-stopping layer 17.
Referring to FIG. 4, the interlayer insulation layer 19 and the etch-stopping layer 17 are continuously patterned to form a contact hole 23 which exposes the metal silicide layer 7 in the gate pattern 8 and the neighboring source/drain region 13.
Contact hole 23 will be filled with a metal to form a contact plug. It is therefore desired to maximize that portion of the surface area of the LDD-type source/drain regions 15 exposed by the contact hole. But the presence of gate spacer 11 limits the exposable area. A further problem arises from possible misalignment during a photo-lithography process. The contact hole 23 can be offset, which further reduces that portion of the surface area of the LDD-type source/drain regions 15 exposed by the contact hole 23. In the worst case scenario, the source/drain region is not revealed at the bottom of the contact hole 23.
When contact area is reduced, the contact resistance increases, interfering with normal operation of a device. This may result in degradation of the performance of the semiconductor device, or it may induce operation failure.
Additionally, as the width of the exposed surface of the source/drain region becomes smaller than the height of the gate pattern in the contact hole, the aspect ratio increases. This increases the difficulty of filling the contact hole with a barrier metal layer and a plug metal layer. This may result in formation of a void at the contact location on the source/drain region, thereby causing an operation failure and degrading reliability in a semiconductor device.
Conversely, using a conventional technique in which spacer thickness is reduced to increase contact exposure to the source/drain region, results in restricting the length of the lightly doped drain region which may result in degradation of transistor performance.